> As with all of these discussions: what costs?
I'll use one of the most cited papers on migration (https://www.jstor.org/stable/24739100) (part of the text here: https://www.oecd.org/els/38195773.pdf). It's a longitudinal study on Denmark during the period of 1991-2008, exploring the results of a wave of new refugees in the country.
The tl;dr is that low-skilled natives were effectively pushed out of their previous employment. They could not, or would not, compete with the migrants owing to ostensibly lower wages and worse working conditions. Instead, they leveraged Denmark's generous free education system, and earned degrees. This afforded them higher wages over a long period of time.
So, one of the costs of high migration - particularly low-skilled migration - is that the poorest lose their jobs, or earn less, or have to work in worse conditions. If you live in a country without free university education, these people will have to contend with a permanent quality of life downgrade.
Then there is housing. The West has experienced enormous pressure on rental and home prices over the last couple of decades. Part of the reason for this is high migration. People need to live somewhere. In many places, for various reasons, production has not kept up. This means locals are forced to pay more for accommodation. This, once again, disproportionately impacts the poor.
I also cite demand for social services and infrastructure, which also appears to be failing to expand at sufficient pace in many nations.
> And how does this differ from the costs of children being born and coming of age into the workforce? Should we in fact be enforcing birth, schooling and university quotas to limit the rate at which new humans can enter the skilled workforce?
Your question implies no distinction between locals and immigrants. That is clearly incorrect. A nation exists to protect the wellbeing of its citizens, not citizens of other nations.